The Clean Air Act of 1970 designated 7 "criteria" pollutants-sulfer dioxide, carbon monoxide, lead, particulates, hydrocarbons, nitrogen oxides, and photochemical oxidants.

Sulfer dioxide is a colorless corrosive gas that is released mainly by stationary fuel combustion(such as a powerplant that burns coal)and industrial processes(smelting of sulfide ores). Sulfer dioxide and sulfer ions are said to be 2nd only to smoking when it comes to air pollution related health damage.

Carbon monoxide is a primary pollutant(in a harmful form when released). It is colorless, oderless, and non-irritating but very toxic. About 67% of the CO in the air is transportation related.

Nitrogen Oxides are examples of secondary pollutants(they become harmful after mixing with the air). They react with the air to form a reddish brown gas, commonly known as smog, and react with water in the atmosphere to make acid rain. About half of the nitrogen oxides in the air are from Transportation and about half from stationary fuel combustion.

Particulates would include dust, ash, soot, lint, spores, smoke, pollen, and algal cells. 32% of the particulates in the air are released by stationary fuel combustion, 32% from industrial processes, 19% from solid waste, and 17% from transportation.

Hydrocarbons, also known as VOC's or volatile organic compounds, are another example of secondary pollutants because they oxidize in the air to form carbon monoxide and carbon dioxide. The majority of the VOC's in the air are from plants, but we also contribute about 28 million tons a year here in the US.

Lead is a metabolic poison and a neurotoxin that is released by fuel combustion and disposal of waste.

As you can see transportation, stationary fuel combustion, and industrial processes are really the main sources for air pollution.